The present invention pertains generally to quantum cascade lasers (QCL), and more particularly, it relates to quantum cascade lasers that operate in the terahertz region of the electromagnetic spectrum.
The terahertz region (e.g., ˜1-10 THz, corresponding to a wavelength λ=30-300 μm or a photon energy ℏω≈4-40 meV) of the electromagnetic spectrum falls between microwave/millimeter and near-infrared/optical frequency ranges. Numerous coherent radiation sources have been developed in the microwave/millimeter and near-infrared/optical frequency ranges. However, despite potential applications of terahertz radiation in a variety of different fields (e.g., spectroscopy in chemistry and biology, plasma diagnostics, remote atmospheric sensing and monitoring, and detection of bio- and chemical agents and explosives for security and military applications), coherent radiation sources operating in the terahertz region remain scarce. The difficulties in developing such radiation sources can be appreciated by considering that semiconductor devices, such as, Gunn oscillators, or Schottky-diode frequency multipliers, that utilize classical real-space charge transport for generating radiation exhibit power levels that decrease as the fourth power of radiation frequency (1/f4) as the radiation frequency (f) increases above 1 THz. Further, the radiation frequencies obtained from photonic or quantum electronic devices, such as laser diodes, are limited by the semiconductor energy bandgap of such devices, which is typically higher than 10 THz even for narrow gap lead-salt materials. Thus, the frequency range below 10 THz is not accessible by employing conventional semiconductor laser diodes.
Some unipolar quantum well semiconductor lasers operating in the mid-infrared portion of the electromagnetic spectrum are known. For example, electrically pumped unipolar intersubband transition lasers, commonly known also as quantum cascade lasers, operating at a wavelength of 4 microns were developed at Bell Laboratories in 1994. Since then, major improvements in power levels, operating temperatures, and frequency characteristics have been made for mid-infrared QCLs.
In contrast to such developments of QCL's in the mid-infrared range, the development of terahertz quantum cascade lasers in a frequency range below 10 THz has been considerably more challenging. In particular, small separation of lasing energy levels (about 10 meV), coupled with difficulties associated with mode confinement, at these frequencies contribute to challenges in developing such lasers.
Hence, there is a need for coherent terahertz radiation sources, particularly, coherent sources that generate radiation in a frequency range of about 1 to about 10 THz.
There is also a need for efficient methods for mode confinement in such terahertz lasers.